Method of producing crystals



April 17, 1956 s. M. THo'MsEN METHOD oF PRoDUcrNG CRYSTALS Filed March 31, 195s INI/ENTOR.

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BY @Qian-f TTORNEY METHOD F PRODUCING CRYSTALS Soren MrTllomsen, Pennington, N. I., assignor to Radio Corporation of America, a corporation of Delaware Application March s1, 1953, serial No. 345,825 6 claims. (ci. 252-501) This invention relates to a method of producing crystalsand moreparticularly to a method of producing cadmium sulphide and cadmium selenide crystals with predetermined electrical properties.

. Cadmium sulphide and cadmium selenide crystals can be produced in a variety of ways. One method is to react cadmium metal in gaseous form with a gas containing sulphur or selenium at a temperature below the sublimation temperature of the reaction product, and allowingcrystals of the reaction product to grow in the region where the reaction takes place.

Crystals made by any of the known methods appear to be alike.A However, when these crystals are examined for electrical properties, their electrical conductivity and photosenstivity 'is found to vary from crystal to crystal. Electrical conductivity refers to the ability of a crystal to pass on electric current and is used as defined by the AIE. This conductivity, when measured for an illumihated crystal, is referred to as the light conductivity of the crystal; and,fwhen measured for vthe same crystal in darkness is referred to as the dark conductivity of the crystal. Photosensitivity, as vused in this application, is the ratio of the. light conductivity to the dark conductivity.

Since the dark and light conductivities and the photosensitivity vary from one crystal to another, it is desirable to provide a method for producing cadmium sulphide and cadmium lselenide crystals that have -predetermined conductivities and photosenstivity.

An object of this invention is to provide an improved method for producing crystals of cadmium sulphide and cadmium selenide.

Another object of this invention is to provide a method for producing cadmium sulphide and cadmium selenide crystals having a predetermined electrical conductivity.

Another object of this invention is to provide a method for producing cadmium sulphide and cadmium selenide crystals having a predetermined photosenstivity.

Another object of this invention is to provide a method for reducing the conductivity of cadmium sulphide and cadmium selenide crystals.

Another object of this invention is to provide a method for increasing the photosenstivity of cadmium sulphide and cadmium selenide crystals.

In general, the present method consists of growing cadmium sulphide or cadmium selenide crystals by one of the prior art methods, and then introducing cations, selected from the group consisting of silver and copper, into the lattice of these crystals. This may be accomplished by immersing crystals of cadmium sulphide or cadmium selenide in a medium containing the desired cations. Then, after removal from said medium and drying, diffusing the cations into the crystals.

The novel features of the invention, both kas to its organization as well as additional advantages thereof, will be set forth in greater detail in the following description in conjunction with the accompanying drawings, in which:

Figure l is a graph showing change of electrical conductivity and photosenstivity of a typical cadmium sul- I 2,742,438 patented Apr. 17,1956

Curve 22shows how the dark conductivity of the crystal -will decline sharply and then level oi when the amount of copper in the crystal isrincreased. Curve 24,shows how the light conductivity of the crystal will decline sharplyvand then level oi when the amount of-copper in the crystal is increased.- YThe photosenstivity, or ratio of light conductivity to ldark conductivity, is shown by curve 26. Since the sharp breaks in curves 22 and 24 do not occur forY the same additions of copper, curve 26 shows a sharp rise and then asharp decline.` Note that conducting crystals treated with small or no amounts of copper remain relatively conducting and exhibit a small photosenstivity. Additions of copper to the lattice of the crystal decreases the conductivity of the crystal but increases the Yphotosenstivity up toan `optimum amount P. Further additions of copper decrease the photosenstivity and decrease the conductivityofthe `crystal further to render the crystal relatively insulating. v

The phenomenon described is'not clearly understood nor has a completely'satisfactory theory been advanced.

It can be seen from these curves that, starting with conducting-type cadmium sulphide crystals, additions of a desired amount of copper will yield crystals having predetermined electrical ltcharacteristics. In accordance with the present invention a, preferred method of introducingsilver `or copper ions 'into'I ,the'lattice of crystals of cadmium sulphide orcadmiumselenide is shown schematically'in Figure 2. This method includes the stepV of immersing a lweighed amount vof crystals in an aqueous solution containing a known amountofcopper or' silver ions. The copper or Isilver ions will replace the cadmium ions on the surface of the crystals. When the replacementis complete, the next step is to wash and dry the crystals, and then heat the crystals to a temperature sufficient to diiuse the copper or silver ions into the crystals. v

In order to incorporate about 5 parts per million of copper into cadmium sulphide crystals, the following procedure may be usedplmmerse 6 grams of cadmium sulphide crystals in 50 milliliters of 0.00001 molar aqueous copper nitrate solution. Agitate the mixture gently for 5 minutes, decant the liquid and wash the crystals twice in triple-distilled water. Dry the crystals in air below C. Fire the crystals at about 700 C. for about 20 minutes in an atmosphere of hydrogen. Cool to a temperature below 150 C. in the same atmosphere before exposing to air. This will yield about 6 grams of cadmium sulphide crystals having about 5 parts per million of copper incorporated in its lattice.

In the above described process, any soluble copper solution of any concentration that will deposit a known amount of copper ions on a known weight of the cadmium sulphide crystals may be used. In the case of incorporating copper or silver into cadmium sulphide crystals, it is preferred to use an aqueous solution of copper or silver nitrate respectively of about 10`7 to about 10-5 molar concentration. In the case of incorporating copper into cadmium selenide crystals, it s preferred to use an aqueous solution of copper nitrate of about 10-4 to about 10-2 molar concentration.

While hydrogen is given as the atmosphere in which to tire the crystals, inert gases such as nitrogen, helium and argon will work equally well. Certain reducing gases such as hydrogen and hydrogen sulphide are satisfactory. The temperature to which the crystals are heated is not rated in cadmiumfsulphidecrystals, copperincorporated in i cadmium selenide crystals, and silver incorporated in cadmium selenide crystals. f

For 4each iof the `above ymentioned materials, `crystals having a conductivity `greater than 103-ohm-cmr'1 are considered tofbe conducting-typecrystals. This would include cadmium sulphide crystals? believed to -contain less than two parts of copper or silver per `million parts of cadmium suphide. Crystals having `a conductivityiess than 10-lo ohm-cm."1 are considered to be-insulating-type crystals. This `would include-cadmium ysulphide crystals believed to contain more than `7 parts of copper or silver per million parts of cadmiumsulphide. Those `crystals that have a conductivity between'l01 ohm-cm.-l and l03 ohmcm.1'have an intermediateconductivity with a high photosensitivity, and would include cadmium sulphide crystals believedto contain 2 to 7 parts of copper or silver per million parts of cadmium sulphide.

What is 4claimed is l. The processof reducing'the electrical conductivity f of crystals of a material selected-from the (group consisting of cadmium sulphide "andrcadmium selenide which comprises immersing'sai'd crystals'inian aqueous solution containing cations selected from-the'group consisting of copper and silver, and heatingsaid'crystals toa temperature sutiicient to'difiuse said-cations into said crystal.

2. The process of `reducing the `electrical conductivity of crystals of a material selected `from the group consisting of cadmiumsulphideand cadmiumv selenide which comprises immersing said crystals inranaqueous solution containing cations selected'fromthe group consisting of copper and silver; vwashingsaid crystals; drying said crystals; and heating said-crystals'toabout 700 C. in a non-oxidizing atmosphere until saidcations are diffused into said crystals.

3. The processof reducing the Velectrical conductivity of crystals of materials selected from the group consisting of cadmium sulphide and cadmiumselenide which comprises immersing said crystals in an aqueous solution containing cations selected frorn the group consisting of copper and silver; washing said crystals; drying said crystals; and heating said crystals to about 700 C. in an atmosphere of hydrogen until said cations are diffused into said crystals.

4. The process of reducing the electrical conductivity of crystals of cadmium sulphide which comprises immersing said crystals in an aqueous solution of coppernitrate of about l0-7 to about l05 molar concentration; washing said crystals; drying said crystals at about 150 C.; and heating said crystals at about 700 C. in an atmosphere of hydrogen until said cations are ditused into said crystals.

5. The process of reducing the electrical conductivity of crystals of cadmium sulphide which comprises immersing saiderystals in an aqueous solution'of silvernitrate of about l0 to about 10*5 molar concentration; washing said crystals; drying `said crystals at about 150 C.; and heating said crystals at about 700 C.fin an atmosphere of hydrogen until said cations are difusedinto said'crystals.

6. The process of reducing theelectrical conductivity of crystals of cadmium selenide which comprises immcrsing said crystals in `an aqueous solution `of `copper -nitrate of about l0r4to about 10-2 molar concentration; washing said crystals; drying said crystals-at about 150 C.; and heating said crystals at about 700 'C. in an'atmospherc of hydrogen until said cations are ditused into said crystals.

References Citedin the file of this `patent UNlTED STATES `PATENTS 2,623,358 Kroger Demo, 195,2 2,651,700 Gans sept. s, 195s Jacobs Apr. ,19, -1955 i OTHER REFERENCES 

1. THE PROCESS OF REDUCING THE ELECTRICAL CONDUCTIVITY OF CRYSTALS OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF CADMIUM SULPHIDE AND CADMIUM SELENIDE WHICH COMPRISES IMMERSING SAID CRYSTALS IN AN AQUEOUS SOLUTION CONTAINING CATIONS SELECTED FROM THE GROUP CONSISTING OF COPPER AND SILVER, AND HEATING SAID CRYSTALS TO TEMPERATURE SUFFICIENT TO DIFFUSE SAID CATIONS INTO SAID CRYSTAL. 